[FFmpeg-devel] r9017 breaks WMA decoding on Intel Macs

Guillaume POIRIER poirierg
Wed May 30 23:35:59 CEST 2007


On 5/30/07, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Wed, May 30, 2007 at 02:07:19PM +0200, Guillaume POIRIER wrote:
> > On 5/30/07, Zuxy Meng <zuxy.meng at gmail.com> wrote:
> > > 2007/5/30, Guillaume POIRIER <poirierg at gmail.com>:
> > > > On 5/30/07, Trent Piepho <xyzzy at speakeasy.org> wrote:
> > > No it won't. Trent's patch is the correct and optimal way, giving gcc
> > > more freedom in allocating general registers. I should have done this
> > > in my original code but I was a bit too lazy and was concerned if too
> > > many constraints would break gcc 2.95, while the fact is Trent's patch
> > > compiles with gcc 2.95. So there isn't any doubt in the patch itself.
> >
> > Ok, fine with me. Michael, do you think that the patch I posted
> > earlier (100% based on Trent's, only fixing minor issues) should be
> > applied?
> well these arent the only occurances of this syntax in ffmpeg

Right. I must admit I didn't see them at first, but a more attentive
grep showed some in libavcodec/i386/fdct_mmx.c,
libavcodec/i386/idct_mmx_xvid.c, libavcodec/i386/simple_idct_mmx.c
(maybe there's more).

I may be wrong, but these seem to implement routines that aren't as
often used as mpeg4, vorbis, mp3 decoding (that's just a wild guess
based on the name of these files) so maybe these are already broken on
MacIntel, but no one noticed.

> also i would like to see benchmarks, gcc has the tendency to do stupid
> things if it can and here it can ... (=more freedom with gcc generally means
> worse code, thats just my experience with gcc, its not always true, also
> gcc should be getting better as the version numbers increase ...)
> also i dont see how this additional freedom could lead to better code
> here, it can just lead to worse code if gcc doesnt realize that things
> can be addressed via the same register

Well, the resulting code is very similar once you look at the nmdiff
(more about this on another mail).

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  -- Renaud (sur "Miss Maggie")

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